Shaft supported rotatable machine members are usually stabilized against axial and radial thrust forces by bearings supporting one or both shaft ends. Prior art bearings, whether of the shaft rubbing or magnetic non-contacting type, generally provide stability by restraining five degrees of shaft freedom. These are radial dislocation of the shaft with respect to mutually orthogonal X and Y axes, tilting of the shaft with respect to either of the X or Y axes, and axial shifting of the shaft along its longitudinal axis. Neither type of bearing is particularly well suited, however, to suspension of slowly rotating shafts such as those driven through oscillating rotational cycles or over arcs of less than one complete turn per cycle.
Shaft rubbing type bearings, such as ball and race bearings or slip ring bearings require periodic lubrication to prevent microwelding or debris accumulation. Adequate liquid lubrication cannot be assured over periods of time on the order of years for shafts operated at low speeds, particularly in an operational environment such as outer space where an ambient vacuum causes liquid lubricants to evaporate. Dry film type lubricants, such as graphite and molybdenum disulfide, have hygroscopic tendencies and leave deposits of debris after a prolonged period of time, characteristics which eventually impair shaft rotation of a machine member supported by the bearing.
Previous efforts to avoid these disadvantages have focused on exotic lubricants and upon non-contacting type rotating bearings such as magnetic bearings. Magnetic bearings avoid the disadvantages of shaft rubbing bearings by eliminating dynamic contact between the shaft and the bearing. Reliable performance of presently available magnetic bearings, however, is typically dependent upon continuous, error-free operation of shaft position sensing and servo-control electronic networks. Implementation of such networks is quite recent and their lack of demonstrated reliability has impeded somewhat acceptance of magnetic suspension bearings by engineers and machine designers. Furthermore, some difficulties have been encountered in combining permanent magnet biased shaft bearings with electric motors for driving a shaft. A large, negative radial force gradient occurs unless the permanent magnet bearing assembly and the iron motor armature are perfectly concentric. Even slight eccentricities create a net radial decentering force due to the forces of attraction between the permanent magnet assembly and the motor armature. Previous efforts to avoid decentering forces have used different flux paths for the motor field and the magnetic bearing assembly thereby incurring the volume and mass of some additional ferromagnetic material necessary to provide the extra flux path.